1. Field of the Invention
This invention relates to a hub assembly for a bicycle, more particularly to a hub assembly with a ratchet-and-pawl mechanism to drive rotation of a hub shell in a uni-direction.
2. Description of the Related Art
In U.S. Pat. No. 7,121,394 B1, the applicant discloses a bicycle hub assembly that includes a hub shell mounted rotatably on a hub axle, a ratchet-and-pawl mechanism disposed inwardly of the hub shell, a chainring carrier coupled with one end portion of the hub shell through the ratchet-and-pawl mechanism, and an annular shielding unit disposed inwardly of the hub shell to shield the ratchet-and-pawl mechanism. The ratchet-and-pawl mechanism includes a plurality of pawls received in retreat bores formed in the end portion of the hub shell, and a surrounding drive member with a ratchet segment that is inserted into an insertion opening formed in the end portion of the hub shell so as to be coupled with the pawls such that the drive of the chainring carrier is transmitted uni-directionally to the hub shell.
The annular shielding unit has a rim which is sleeved on the chainring carrier, and a surrounding portion which extends radially from the rim towards the insertion opening so as to be kept in frictional contact with an inner peripheral surface of the end portion of the hub shell. However, dust and moisture may enter the clearance between the surrounding portion and the chainring carrier.